AQA GCSE Chemistry Unit 3 – Quantitative Chemistry

A set of flashcards covering key definitions, equations and examples for AQA GCSE Chemistry Unit 3: Quantitative Chemistry, including relative masses, mole calculations, concentrations, gas volumes, yields and atom economy.

What does the relative atomic mass (Ar) of an element compare?

It compares the mass of an atom of the element to 1/12th of the mass of a carbon-12 atom.

How is relative formula mass (Mr) calculated?

By summing all the relative atomic masses of the atoms present in a compound’s formula.

What is the Mr of HCl?

36.5 (1 for H + 35.5 for Cl).

Calculate the Mr of H₂SO₄.

98 [(1×2) + 32 + (16×4)].

Do Ar and Mr have units?

No, they are dimensionless numbers.

State the law of conservation of mass.

No atoms are lost or made during a chemical reaction; total mass of reactants equals total mass of products.

Why can mass appear to change in a non-enclosed system?

Because a gas may enter or leave, making the measured mass seem to increase or decrease.

Define ‘uncertainty’ in measurements.

A numerical estimate of the range within which the true value is expected to lie.

Give one cause of random error.

Human reading errors or small fluctuations in faulty equipment.

Give one cause of systematic error.

Incorrectly zeroed balance or a consistently flawed experimental method.

How is the range of a data set calculated?

Largest value minus smallest value.

How is uncertainty of the mean estimated from a range?

Uncertainty = range ÷ 2.

Write the equation for concentration using mass.

Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).

Convert 250 cm³ to dm³.

0.250 dm³ (divide by 1000).

What is a mole in chemistry?

The amount of substance containing 6.02 × 10²³ particles.

State Avogadro’s constant to 3 significant figures.

6.02 × 10²³.

How is the mass of one mole of a substance related to its Mr?

The mass in grams of 1 mole equals the substance’s Mr.

Write the formula linking number of particles (N), moles (n) and Avogadro's constant (NA).

N = n × NA.

Calculate the number of particles in 0.25 mol of any substance.

0.25 × 6.02 × 10²³ = 1.505 × 10²³ particles.

What is meant by a limiting reactant?

The reactant that is completely used up first and thus limits the amount of product formed.

What is a reactant ‘in excess’?

A reactant left over after the reaction stops because another reactant has been used up.

List the three key steps to find maximum product mass from a limiting reactant.

a) Balance the equation; b) find moles of limiting reactant; c) use mole ratio and Mr to find product mass.

Which equation is used to balance Mg and O₂ if 12 g Mg reacts with 8 g O₂?

2Mg + O₂ → 2MgO.

Give the concentration formula using moles.

Concentration (mol/dm³) = moles ÷ volume (dm³).

How do you convert concentration from mol/dm³ to g/dm³?

Multiply by the Mr of the solute.

How many grams of NaOH are in 0.8 mol/dm³ solution? (Mr 40)

32 g/dm³ (0.8 × 40).

What volume of solution (dm³) is produced when 8.80 g of solute makes a 42 g/dm³ solution?

0.210 dm³ (mass ÷ concentration).

Give the formula for amount of solute when concentration and volume are known.

Moles = concentration (mol/dm³) × volume (dm³).

How many moles are in 75 cm³ of 0.20 mol/dm³ NaOH?

0.015 mol.

State the percentage yield formula.

% yield = (actual mass / theoretical mass) × 100.

If 1.8 g is obtained but 2.0 g was expected, what is the percentage yield?

90 %.

Give two reasons why yield may be less than 100 %.

Product lost during transfer/filtration OR reaction not going to completion (reversible, side reactions, etc.).

State the atom economy formula.

Atom economy = (Mr of desired product ÷ Σ Mr of all reactants) × 100.

What is the atom economy when fermenting glucose to ethanol (Mr C₆H₁₂O₆ = 180, desired 2×46)?

51.1 %.

Why is high atom economy desirable?

It reduces waste, conserves resources and improves economic and environmental sustainability.

Name five factors influencing choice of reaction pathway.

Percentage yield, atom economy, rate of reaction, equilibrium position, usefulness of by-products.

State the molar gas volume at room temperature and pressure (rtp).

24 dm³ per mole of gas.

Write the formula linking gas volume, moles and molar volume.

Volume (dm³) = moles × 24 dm³ mol⁻¹.

What volume does 0.55 mol of CO occupy at rtp?

13.2 dm³.

State Avogadro’s law in terms of gas volumes.

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

How many moles of hydrogen gas occupy 198 cm³ at rtp?

0.0083 mol (198 cm³ ÷ 24 000 cm³ mol⁻¹).

Give the balanced equation when sodium reacts with water.

2Na + 2H₂O → 2NaOH + H₂.

If 3.5 g Na reacts, how many moles of H₂ are produced?

0.075 mol (0.15 mol Na × ½).

Calculate the volume of that hydrogen at rtp.

1.8 dm³ (0.075 mol × 24 dm³ mol⁻¹).

Write the formula for mass from moles and Mr.

Mass (g) = moles × Mr.

How many moles are in 330 g of K₂S?

3 mol.

Which reactant limits production of MgCl₂ when 36 g Mg reacts with excess HCl?

Magnesium (because HCl is in excess).

What maximum mass of H₂ is produced from 36 g Mg?

3 g.

Describe how to convert m³ to dm³.

Multiply by 1000.

Provide the units for concentration when using mass and when using moles.

g/dm³ (mass) and mol/dm³ (moles).